1. Field of Invention
This invention relates generally to the production of blocks of open-cell, flexible polyurethane foam material which can then be sectioned into useable slabs for cushioning and other useful applications, and more particularly to a system for producing these blocks and for conditioning each freshly-manufactured block to impart thereto optimal flexible foam properties.
2. Status of Prior Art
Flexible, open-cell polyurethane foam slabstock is widely used in upholstered furniture, mattresses, carpet underlays and in many other useful applications which require cushioning or shock absorbing properties. It is important therefore that manufactured blocks of this material from which the slabs are sliced have a predetermined density that is uniform throughout the block, that the IFD (indentation force deflection) characteristics of the material be consistent from block to block, and that in general, the properties of the open-cell, flexible foam material satisfy existing criteria for the intended applications.
Various processes for manufacturing large blocks of open-cell, flexible polyurethane foams are well-known. These blocks can be produced on a batch basis in a mold, or continuously in a conveyor-type machine. In this machine, the flowable constituents of the foam are poured into a conveyor inlet station, the constituents interacting to create on the conveyor a large block which is transported to storage or other facilities.
Flexible polyurethane foam is formed by a reaction between a high molecular weight polyol, a diisocyanate and water in the presence of a surfactant. Polyurethane foam commonly contains butylate hydroxy-toluene ("BHT") which is used as an antioxidant in the polyols that are reacted with isocyanates, such as toluene diisocyanate ("TDI"), to form the foam.
This reaction is highly exothermic, reaching a peak, as depicted in a time/temperature curve, typically within about 5 to 30 minutes. Polyurethane foam buns or blocks therefore have to be transferred to a "cure area" where they are carefully placed with an air space around each block until they have cooled. A large area is required for this purpose and the blocks must be stored for a minimum of 10 hours or for a much longer period before they can be restacked or loaded for delivery to a customer.
This process of intermediate storage to ensure adequate cooling of the blocks is inconvenient and costly in terms of space and handling requirements. Moreover, the intermediate storage area contains a large number of blocks of inflammable foam at high temperature, presenting a potential fire hazard. Hence buildings used for such intermediate storage need to be specially constructed to meet fire regulations.
The reason a freshly-manufactured block placed in a storage area in a relatively cool environment assumes a high internal temperature is that the block undergoes two exothermic reactions. The initial reaction takes place within a few minutes after the block is freshly manufactured, the internal temperature of the block then rising to a level of about 180 to 380 degrees Fahrenheit, depending on the water content of the foam, after which it declines. But about an hour and a half later, a second exothermic reaction transpires causing the internal temperature to rise to a level of 300 to 350 degrees Fahrenheit, or even to a point of auto-ignition, depending on the water content of the foam. This high temperature causes discoloration or scorching of the block and in other respects degrades its desirable properties.
Scorching is a result of an oxidation reaction with ambient air which infiltrates the block following expulsion of CO.sub.2 produced during the foam-forming reaction. This oxidation reaction which takes place with unreacted isocyanate and the hot foamed polymer is undesirable for several reasons. Even at low levels, such oxidation can produce yellowing of light colored foams, which can render the foam product unacceptable for some commercial uses. If the oxidation reaction is excessive, as evidenced by more than slight yellowing, a deleterious effect on the physical properties of the foam will result. This undesired oxidation reaction can proceed with sufficient intensity to actually cause the foam to ignite and burn.
To overcome these drawbacks, the prior art has long recognized the desirability of a process for the rapid cooling of freshly-manufactured blocks of open-cell, flexible foam polyurethane material.
Thus the Ricciardi et al. U.S. Pat. No. 3,890,414 discloses a forced-air cooling process in which a freshly polymerized foam bun is cooled by passing a cooling gas through the foam mass, a vacuum being applied to one surface of the bun. The rapid cooling process disclosed in this patent reduces the amount of time required to cool foam buns or blocks and produces a product with more uniform properties.
A disadvantage of the Ricciardi process is that the gas exhausted from the foam contains particulate matter generated by the exothermic reaction, as is evidenced by a smoke plume. This smoke which is exhausted into the atmosphere, is not environmentally acceptable.
A later Ricciardi U.S. Pat. No. 5,171,756 discloses a three-stage cooling process in which in the first stage, cool air is drawn through the foam body and then exhausted into the atmosphere. This patent indicates that the exhausted air contains excess water, BHT and a minor proportion of TDI ureas. This air is exhausted to the atmosphere to prevent BHT and TDI ureas from clogging heat exchangers. In stage two, sublimates are withdrawn from the foam with air that is subsequently cooled to condense the sublimates, and is then recirculated through the foam to redeposit the sublimates uniformly throughout the foam. In stage three, additional cool air is drawn through the bun and is vented to the atmosphere to remove moisture and volatile components.
In the patent to Stone U.S. Pat. No. 5,128,379, rapid cooling is effected by passing through the porous polyurethane mass a coolant stream having a water content close to the dew point, the cooling stream being recirculated.
Prior art rapid cooling systems for open-cell, flexible polyurethane foam blocks do not adequately deal with the excessive heat caused by a second exothermic reaction which takes place in the manufactured block. An initial exothermic reaction is experienced within a few minutes after the block is freshly manufactured, but the peak temperature of this reaction is not sufficient to cause scorching of the foam or auto-ignition. But after a relatively long period of two to three hours, reaction products, such as hot fumes produced during the first exothermic reaction react with air drawn into the open-cells of the block to produce a second exothermic reaction having a considerably higher peak temperature. It is this second exothermic reaction that results in scorching of the foam and in the degradation of its useful properties.
With rapid cooling systems of the type heretofore known, the hot fumes and the particulate matter entrained therein generated during the first exothermic reaction are not fully evacuated from the block by cooling air passing through the block. The reason why pockets of these hot fumes, entrained particulate matter and other contaminants remain entrapped within the porous block is that the block is naked and exposed to the atmosphere. The cooling stream forced through the porous block is not distributed throughout all internal regions of the block and does not therefore fully evacuate from the block these contaminants to obviate a second exothermic reaction.